Therefore the cam gear ratio must be as one is to two, the smaller gear being placed on the crankshaft, the gears being known as the “half time gears.”
As a two stroke cycle engine goes through the routine of events in every revolution, the cam-shaft must run at crankshaft speed so that the cam outline makes one revolution in the same time as the crank. The cam shaft speeds given here apply to all engines of the corresponding cycle no matter whether the valves are of the poppet, rotary or slide-sleeve types.
(135) Valve Gear Troubles.
The valve gear mechanism causes trouble principally through the wear of the various parts which results in a change in the valve timing, or in the lift of the valves. Loss of power, MISFIRING, and overheating are the result of such derangements.
Often trouble is caused in reassembling the valve mechanism after the engine has been torn down for repairs, which trouble may generally be traced to incorrect gear meshing.
The following list will give the principal defects due to the wear of the valve mechanism.
(a) WORN CAM GEARS change timing because of play, or “back lash” in the teeth, or cause a howling or grinding noise, that will cause the owner to believe that the end of the engine is near. MISFIRING and LOSS of power are probable results of a change in the timing. If any of the teeth are stripped from the gear you may be sure that the timing is changed. Replacement with a new gear is the only cure for a worn or broken gear.
(b) GEARS NOT IN PROPER MESH due to an error in assembling the gears, will prevent the engine from being started, or cause misfiring and loss of power.
The maker of the engine generally marks the teeth that go together, but if no such marks appear, the owner should center punch or scratch them before taking down the engine.
(c) A GEAR SLIPPING ON THE SHAFT, due to a missing key in the gear, or to a loose set-screw will cause all of the troubles due to a change in the timing. Examine the key carefully, for dirt often collects in the key-way to such an extent that it is liable to be mistaken for the key. Keys and pins have sheared in two, allowing the shaft to slip in the gear.